Potential role of P-gp for flavone-induced diminished apoptosis and increased adenoma size in the small intestine of APC(min/+) mice.

APC(min/+) mice, carrying a nonsense mutation in the adenomatous polyposis coli (APC) gene, appear as a perfect model to study development or therapy of intestinal neoplasia. We tested whether the flavonoid flavone is able to affect adenoma development in APC(min/+) mice. Tumor sizes were significantly increased by flavone selectively in small intestine. This was associated with reduced cell numbers displaying cleaved caspase-3 and enhanced expression of phosphoglycoprotein (P-gp). However, according to great variability in P-gp expression in all parts of mice intestines, an association between expression of P-gp and inhibition of apoptosis was demonstrated in human Caco-2 colorectal cancer cells.

Chrysin blocks topotecan-induced apoptosis in Caco-2 cells in spite of inhibition of ABC-transporters.

ATP-driven efflux pumps such as phosphoglycoprotein-170 (P-gp), multidrug-resistance-associated protein-2 (MRP-2), or breast cancer resistance protein (BCRP) play a crucial role in limiting the efficacy of tumor pharmacotherapy. Selected flavonoids have been suggested to inhibit individual efflux-transporters and to act therefore as multidrug-resistance reversing agents. In the present study it is shown that the flavonoid chrysin acts as a potent inhibitor of P-gp, MRP-2, and BCRP in Caco-2 colon carcinoma cells. As a consequence, cells accumulated higher rates of the apoptosis-inducing chemotherapeutic topotecan in the presence of chrysin, even though under these conditions the expression of the transporters was markedly increased. Interestingly, in spite of the enhanced cellular drug accumulation the topotecan-induced apoptosis, assessed according to DNA-fragmentation, chromatin condensation, and by determination of sub-G1 peaks using fluorescence-assisted-cell sorting (FACS), was potently inhibited by chrysin. Suggested transport-independent apoptosis inhibiting activities of ATP-binding cassette (ABC)-transporters, such as the inhibition of caspases, were shown to be necessary for the inhibition of topotecan-induced apoptosis and were found to be associated with stabilization of beta-catenin especially in the cytosol. Inhibition of topotecan-induced intracellular acidification, however, was proven not to prevent caspase-activation and apoptosis. In conclusion, our studies show that chrysin in spite of raising the cellular concentrations of topotecan potently inhibits the apoptosis-inducing activities of the anti-tumor drug. Inhibition of caspase-activation was identified as the underlying mechanism and is suggested to be caused by transport-independent functions of ABC-transporters.